Hydration and Ion Transport Behavior of Ion-Conducting Polymers: Optimizing Vanadium Redox Flow Battery Performance

A greater understanding of the behavior of proton exchange membranes (PEMs) is critical to advance sustainable energy technologies. To improve the performance of PEMs, we need to improve our fundamental understanding of proton-conducting ionomers in membrane separator format. This thesis thus focused on understanding the water uptake, proton (H+) permeation, and vanadium ion (V+n) permeation in ~100 µm-thick bulk, free-standing membranes made from a range of proton-conducting ionomers. This study is critical for vanadium redox flow batteries as in an ideal scenario, the PEM used as separator in a redox flow battery should transport protons efficiently, while blocking the permeation of redox ions (i.e., vanadium ions) effectively. The ionomers chosen for this work were sulfonated polysulfone (sPSf), sulfonated oligosulfone (sOSf) and a proprietary ionomer (P2). All these ionomers are hydrocarbon-based, but vary in molecular weight, chain length, and porosity. The proton permeability of pure Nafion membrane (DH+=1.01´10-6 cm2/min) was higher than pure sPSf membrane (DH+=1.69´10-11 cm2/min), but the V+n permeability of pure Nafion membrane (DNafion,Vn+=9.75´10-7 cm2/min) was also higher than pure sPSf membrane (DsPSf,Vn+=3.52´10-8 cm2/min). The introduction of oligomeric sOSf into the polymeric sPSf matrix in a ratio of 0.05:1 resulted in a slight decrease in vanadium ion (V+n) permeability. The corresponding V+n diffusion coefficients were: 1.76´ 10-8 cm2/min (composite), and 3.52 ´ 10-8 cm2/min (pure sPSf). However, the proton permeation also decreased (DsPSf-sOSf, H+ = 2.64 ´ 10-12cm2/min) which was not desired. On the other hand, the incorporation of P2 into the Nafion membrane matrix led to a decrease in proton permeation (DNafion-P2, H+=4.41 ´ 10-7 cm2/min), but V+n permeation (DNafion-P2, Vn+=2.57 ´ 10-6 cm2/min) was still high. The water uptake data corroborated with proton and V+n permeation characteristics of these membranes. The whole study indicated the challenge of achieving both high proton permeability and low V+n permeability from a single ionomeric membrane